1. Field of the Invention
The present invention relates to an optical disc device for reproducing information by irradiating a laser beam onto an optical disc, and more specifically to a technique of removing a noise component generated by fluctuation of a laser beam from a reproducing signal.
2. Description of the Related Art
In recent years, since information recording density in the optical disc has been intensified, it becomes increasingly difficult to achieve an S/N ratio (signal to noise ratio) necessary for reproducing information. Reproducing signals obtained by irradiating a laser beam onto an optical disk contain various noise components. The noise contained in the reproducing signal can be classified into the following three types by the cause:
(1) Disc noise caused by roughness or inconsistency on the surface of an optical disc,
(2) Circuit-related noise generated in a circuit including a beam receiver and an amplifier, and
(3) Laser beam source noise (LD noise) caused by a laser diode (LD) which is the light source.
The circuit-related noise can be further classified into beam-receiver (PD) amplifier noise generated depending upon a design of a first-stage amplifier of the beam receiver, shot noise caused by fluctuation of electrons in receiving current of the beam receiver, thermal noise due to conversion resistance of the first-stage current-voltage converter and amplifier of the beam receiver, and the other noise. The thermal noise can be obtained theoretically.
The generation of laser beam source noise is due to fluctuation in wavelength and power of the laser beam, and is caused by a transition of emission mode triggered by temperature change, return beam and so on. The most dominant noise is quantum noise caused by fluctuation of electrons in the electric current that flows through the laser diode.
In recent years, there has been developed a technique for removing the laser beam source noise. Specifically, an optical disc device is provided with a reproducing signal beam-receiver which receives a return beam from the optical disc and generates a reproducing (RF) signal, and a beam-source monitoring beam-receiver which monitors the laser beam source and generates an automatic power control (APC) signal. The beam source noise contained in the reproducing signal is removed by extracting a laser beam source noise cancellations (LNC) signal from the beam-source monitoring beam-receiver.
Referring to FIG. 1, a conventional device and method for removing the laser beam source noise will be described. The example to be described was disclosed in the Japanese Laid-Open Patent No. 10-124919 which was the Japanese Patent Application No. 8-276008 filed on Oct. 18th, 1996 by the same applicant as of the present application. For details see the above gazettes.
As shown in the FIGURE, the optics system of this optical disc device includes a laser diode 11 as a beam source, a collimator lens 12, a polarization beam splitter 13, a quarter-wave plate 14, an object lens 15, a condensing lens 16, a reproducing signal beam-receiver 17, and a beam-source monitoring beam-receiver 18. The laser beam from the laser diode 11 is made into parallel beams by the collimator lens 12, and then divided into two beams by a half mirror 13A of the polarization beam splitter 13. One of the divided beams is polarized by the half mirror 13A and then received by the beam-source monitoring beam-receiver 18. The other of the divided beams passes through the half mirror 13A, the quarter-wave plate 14 and the object lens 15, and reaches an information recording surface of an optical disc 20. The beam from the information recording surface of the optical disc is polarized by the half mirror 13A of the polarization beam splitter 13, passes through the condensing lens 16 and then received by the reproducing signal beam-receiver 17.
Now, an output signal from the beam-source monitoring beam-receiver 18 should show a level fluctuation corresponding to fluctuation in a wavelength and power of the laser beam. Specifically, the output signal from the beam-source monitoring beam-receiver 18 should have the same phase as the laser beam source noise contained in an output signal from the reproducing signal beam-receiver 17. Therefore, the laser beam source noise can be removed from the reproducing signal through a cancellation calculation of the output signal from the beam-source monitoring beam-receiver 18 from the output signal from the reproducing signal beam-receiver 17.
An example of the cancellation calculation will be described. The output signal from the reproducing signal beam-receiver 17 is supplied to an accumulator, via a current-voltage converter and amplifier. The output signal from the beam-source monitoring beam-receiver 18 is supplied to the accumulator, via a current-voltage converter and amplifier, after the signal phase is inverted by a phase inverter. The accumulator outputs a reproducing signal from which laser beam source noise has been removed.
As described above, the reproducing signal includes not only the laser beam source noise but also the circuit-related noise such as the beam-receiver amplifier noise, or the like. To reliably perform the canceling operation of the laser beam source noise and to reduce the circuit-related noise are not always achievable simultaneously.
If the circuit or the optics is designed so that the laser beam source noise can be completely removed from the reproducing signal by performing the cancellation calculation between the output signal from the reproducing signal beam-receiver 17 and the output signal from the beam-source monitoring beam-receiver 18, sometimes the result is an increase in the circuit-related noise. Accordingly, if the cancellation calculation yields an increase in the circuit-related noise greater than a decrease in the laser beam source noise, the noise is increased as a result.
Consequently, it is an object of the present invention to provide an optical disc device capable of minimizing the noise contained in the reproducing signal, taking into account not only the laser beam source noise but also the circuit-related noise.